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What will happen if connect an iron rod with a motor's axis and rotate it in space and after a while disconnect the motor's axis from the rod. Will after this the rod show infinity motion and if not why?

stafusa
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Sharpmouse123
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  • I asked tis because as i know inertia helps a body to be in its motion till the time its motion is not disturbed by any other fact factor and in space due to the absence of air or earth's gravity any object if rotated should show infinity motion due to its inertia . Am I correct. – Sharpmouse123 Dec 22 '17 at 06:28
  • Why an iron rod? Are you spinning the rod around its long axis, or are you spinning it like a propeller? It will keep spinning for a very long time, but not forever. – PM 2Ring Dec 22 '17 at 06:43
  • Like a propeller – Sharpmouse123 Dec 22 '17 at 17:25
  • Why do you think you might not be correct? – sammy gerbil Dec 23 '17 at 20:39

1 Answers1

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In the ideal situation in classical mechanics, Newton's 2nd law tells us that in the absence of forces and torques, the rod will have no acceleration, which means by definition that it'll keep moving forever at constant speed.

In reality there are always energy losses, therefore the kinetic energy $K=mv ^2/2$ will inevitably decrease with time. Examples of unavoidable losses are the emission of gravitational waves, which any accelerated non-point object with mass emits, and collisions with small particles (as "even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter" - Wikipedia).

Regardless of the losses, though, the rod could still display endless, if vanishing movement (as in, for instance, $K\propto e^{-at}$, which goes to zero for $t\to\infty$, but is finite for any finite time $t$). For small enough speeds, also the few atoms colliding with the rod would guarantee that complete rest wouldn't be reached. And even if the space could be perfectly empty, I'd expect quantum effects to show up at low enough temperatures and velocities, preventing the rod from being perfectly still (but then, if we want to consider quantum mechanics, space is not empty, but filled with fields).

If the rod were still connected to the motor, inhomogeneities and other imperfections (e.g., a non-uniform friction coefficient) would be likely to bring the motor to a complete stop at some point: in the same way that, when pushing a heavy piece of furniture across a room at approximately constant speed, you might find it getting suddenly stuck at a rough patch or unevenness of the floor.

stafusa
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